a) Field of the Invention
The present invention relates to a viewfinder optical system for silver salt cameras, still video cameras, video cameras, etc., and more specifically Keplerian viewfinder optical system.
b) Description of the Prior Art
Viewfinders are classified into the so-called virtual image type which includes Albada viewfinders and inverted Galilean viewfinders, and the so-called real image type which includes Keplerian viewfinders.
Out of the viewfinders mentioned above, the Keplerian viewfinder consists, as shown in FIG. 1, of an objective lens system O which functions to form an image of a visual field on an intermediate image surface I corresponding to the image formed on the photographing screen by a photographic lens and an eyepiece system E which functions to magnify the image of the visual field as a virtual image. In addition, a relay imaging system for vertically and horizontally inverting the image or an optical system comprising porro prisms, etc is arranged at an optional location in the optical path of the Keplerian viewfinder.
As a conventional example of the viewfinder optical systems used in this type of viewfinder, there is known the optical system disclosed by Japanese Patent Kokai Publication No. Sho 61-156018.
This conventional viewfinder optical system has the composition illustrated in FIG. 2, i.e., consists of an objective lens system O which comprises three lens components of three lens elements and an eyepiece lens system which comprises three lens components of three lens elements. In this optical system, the first lens component L1 of the eyepiece lens system E is lens component for relaying a pupil, whereas the second lens component L2 and the third lens component L3 have positive refractive powers respectively. This optical system is designed in such a manner that an intermediate image I is formed before the eyepiece lens system E and in the vicinity thereof, and prisms for vertically and horizontally inverting the image are to be arranged in the airspace represented by the reference symbol A and the prism section designated by the reference symbol B in FIG. 2. This conventional viewfinder optical system has an eye point set at a location 13.5 mm after the eyepiece lens system E, and the optical system composed of the two eyepiece lens components arranged on the eye point side of the section B locates the front principal point at the position of HF1 and the rear principal point at the position of HB1. The front principal point is located at a position 32 mm as measured from the vertex of the object side surface of the object side lens component L2 in the direction toward the eye point, whereas the rear principal point HB1 is located at a position 2.32 mm as measured from the vertex of the eye point side surface of the eyepoint side lens component L3 in the direction toward the object side. The front principal point HF1 and the rear principal point HB1 are located within the viewfinder optical system.
In the conventional viewfinder optical system described above, the objective lens system has a half field angle of 24.7.degree. on the side of incidence at the wide position and a focal length of 9.975 mm, whereby the intermediate image has a height of 4.59 mm.
When the Rica size of a photographic plane is taken as an example, the ratio of lengths of the plane is:
Diagonal length: length of longer side: length of shorter side=1:0.832:0.555
When either of the two types of porro prisms shown in FIG. 3A and FIG. 3B is used, the prism folds the optical axis of the optical system twice in the direction along the longer side and twice in the direction along the shorter side. When the ray passing through the portion of the prism which is the farthest from the optical axis is nearly parallel with the optical axis, the space required for arranging the prisms must have a length on the optical axis equal to the longer side.times.2+length of the shorter side.times.2.
Calculating on the basis of the intermediate image height in the above-described conventional view-finder optical system and the ratio in lengths on the photographic plane, the length on the optical axis required for arranging the prisms is 25.47 mm. However, the section B of the above-described conventional example has an optical path length as short as 20 mm. Accordingly, the conventional example does not allow the prisms to be arranged only in the section B, and therefore requires arrangement of the prisms in both the airspace A and the section B, or arrangement of prisms in the section B and mirrors in the airspace A.
As is understood from the foregoing description, the conventional viewfinder optical system cannot adopt a composition wherein only a prism having four reflecting surfaces is arranged, whereby the conventional viewfinder optical system is composed of a large number of optical members, retaining members therefor, etc., and requires a high manufacturing cost.
Further, when each of the above-mentioned optical members is composed of a plural number of parts, it is more difficult to set each of the reflecting surface relative to the optical axis with high angle precision than in a case where each of the optical members is composed of a single part.
Though it is considered to prolong the focal length of the eyepiece lens system as a whole for prolonging the optical path lengths of the prisms, such a measure will lower magnification of the viewfinder and makes it impossible to observe images of desirable sizes.